Disazo pigment sensitized photoconductor

ABSTRACT

A photoconductor of the organic type is disclosed where a sensitizing disazo pigment is milled to a submicron particle size in an alkyl substituted benzene or alkyl substituted benzene blended with another solvent resulting in a more light sensitive photoconductor with good stability characteristics.

United States Patent 1191 Kukla Nov. 27, 1973 [5 1 DISAZO PIGMENTSENSITIZED 3,622,341 11/1971 Lee 96/1.6 PHOTOCONDUCTOR 3,384,632 5/1968Solodar 96/1.6 X 3,597,196 8/1971 Jones et a1. 96/].6 [75] Inventor:William J. Kukla, Lexington, Ky.

D FOREIGN PATENTS OR APPLICATIONS [73] Asslgnee' lmemamfnal Busmes; gfi964,873 1 7/1964 Great Britain 96/].6 cm'pm'ammArmm 964,877 7/1964 GreatBritain 96/1.6 [22] Filed: Dec. 26, 1972 Primary ExaminerRoland E.Martin, Jr. [21] Appl 317393 AttorneyLaurence R. Letson et a1.

[52] US. Cl. 96/16, 96/15 57 ABSTRACT [51] Int. Cl 603g 5/06 58 Field 61Search 96/15, 1.6 A the 9 dscmsed where a sensitizing dlsazo pigment ismilled to a sub- 56 R f d micron particle size in an alkyl substitutedbenzene or 1 UNITE]; gig LZ alkyl substituted benzene blended withanother SOI- vent resulting in a more light sensitive photoconductor3,484,237 12/1969 Shattuck et a1 96/1.5 with good stabilitycharacteristics 3,287,121 11/1966 Hoegl 96/1.5 3,634,079 H1972 Champ96/l.5 9 Claims, No Drawings DISAZO PIGMENT SENSITIZED PHOTOCONDUCTORRELATED CO-PENDING APPLICATION Electrophotographic Process Using DisazoPigments, Ser. No. 129,635, filed Mar. 30, 1971, inventors R. B. Champand M. D. Shattuck.

BACKGROUND OF THE INVENTION It has been discovered in working with thephotoconductor described in US. Pat. No. 3,484,237 to Shattuck, et al.,Organic Photoconductive Compositions and Their Use inElectrophotographic Processes, that the sensitivity of thephotoconductor may be enhanced when sensitized with a disazo pigmentsuch as is disclosed in the co-pending application referenced above.

It was further discovered that additional light sensitivity was securedwhen the coating was not completely dried and there was a retainedsolvent level in the coating approximating 1.5 percent to 2.0 percent.

Under ambient conditions this retained solvent level or residual solventlevel was not sufficiently high to create an' instability phenomena.However, when this photoconductive structure with a high residualsolvent level was placed in an electrophotographic machine environment,where the photoconductor temperature may reach as high as 140 to 150 F.due to'energy dissipated from electrical components, light beinggenerated by the illumination system, and the heat'energy generated bythe fuser mechanism, the residual solvent in the photoconductor wasdriven off with time and the enhancement of the light sensitivitypresent previously, was at least partially destroyed.

SUMMARY OF THE INVENTION A stable photoconductor may be enhanced inlight sensitivity while obtaining the advantages of unstable solventlevels by milling the sensitizing disazo pigment in an alkyl substitutedbenzene or a blend thereof with another solvent, prior to adding it tothe photoconductor matrix material. This procedure eliminates the needto retain unstable residual solvent levels in the. final photoconductorstructure.

It is an object of the invention to eliminate problems associated withhigh residual solvent levels in organic photoconductive coatings.

Itis a further object of this invention to alleviate the detrimentaleffect on the sensitizing capability of disazo pigments when saidpigments are prepared in 'tetrahydrofuran alone.

It is an additional'object of this invention to enhance the sensitivityof the photoconductive structure comprising apolyvinylcarbazole/2,4,7-trinitro-9- fluorenone charge transfer complexsensitized with a disazo pigment.

The foregoing objects and benefits will be more easily understood fromthe following more specific examples and detailed description.

DETAILED DESCRIPTION It should be understood at the outset that theexact mechanism by which this invention functions is not presentlyunderstood, but that there is aclearly discemable pattern evident whichsupports the thesis that the milling and preparation of a disazo pigmentto submicron sized particles in a solvent selected from the family ofalkyl substituted benzenes, which have a boiling point less than about250 C. and a melting point below 20 C., is advantageous. The subsequentaddition of the pigment paste formed by milling the disazo pigment inthe alkyl substituted benzene to a photoconductive charge transfercomplex of polyvinylcarbazole (PVCz) and 2,4,7, trinitro-9-fluorenone(TNF) results in a more light sensitive photoconductive member comparedto such a photoconductor when the identical materials are prepared in astraight, single solvent system where the single solvent istetrahydrofuran.

It has been found that when a disazo pigment, and more particularly whenchlorodiane blue pigment is added to an approximately 1:1 molar weightratio of an organic photoconductive charge transfer complex,particularly polyvinylcarbazole and 2,4,7 trinitro-9- fluorenone, thelight sensitivity of the photoconductor is greatly enhanced. Furtherlight sensitivity is evident when the solvent from which thephotoconductive layer is coated is not completely dried out of theresulting structure. It has been found that residual tetrahydrofuransolvent levels of about 1.5 percent yield an increased speed or lightsensitivity, however, this level is unstable at elevated temperatures.

The temperature of a photoconductor rises due to the normal operatingtemperatures of an electrophotographic copying machine or apparatus, andthus the residual solvent is driven off with time. When the residualsolvent is evaporated to below about 1 percent, most of the speedadvantage observed as a result of the high solvent level, is dissipated.

Some of the benefits of the high residual solvent level may be obtainedby using a solvent blend where one of the solvents is tetrahydrofuranand the second solvent is an alkyl substituted benzene. The best resultsare obtained when the chlorodiane blue pigment is milled to a submicronparticle size in the alkyl substituted benzene alone and slightly lesseffectively in a tetrahydrofuran/toluene blend.

It is believed that there is a detrimental effect generated on thesensitizing properties of the disazo pigment by the contact of thepigment with other ingredients in the photoconductive structure whentetrahydrofuran solvent is used.

It is believed, insofar as the invention is understood, that the use ofalkyl substituted benzene eliminates or at least minimizes anyundesirable effects on the disazo pigment when the alkyl substitutedbenzenes are used either in a blend with tetrahydrofuran or bythemselves, during the step of milling the pigment paste for addition tothe photoconductive organic solution.

It is believed that thereis a surface wetting of the pigment particleswith the alkyl substituted benzenes and that this tends to act as abarrier between pigment and other materials in the solution/dispersionwhich are insoluble in the alkyl substituted benzenes but otherwiseadvantageous to the final product.

EXAMPLE 1 To an abrasion resistant container is added a quantity of wellcleaned steel shot. To the container and the shot is added chlorodianeblue pigment and toluene, in the proportion of 6 grams of pigment to 94grams of tolu ene. The mixture of pigment, toluene and shot is agitatedon a Red Devil paint shaker for about minutes to mill the pigment to asubmicron size and wet the particles with solvent. The agitation resultsin a pigment paste which is removed from the steel shot and container. Aquantity of pigment paste is added to 250 grams of a 1:1 molar weightratio of PVCz:TNF charge transfer complex solution containingapproximately 8 percent DuPont 49000 adhesive to yield a 6 weightpercent pigment concentration based on total solids. The 1:1 molar ratioPVCz:TNF and 49000 adhesive, are dissolved in tetrahydrofuran. Theamount of pigment paste added depends upon the percent solids of thecharge transfer complex solution. The above blend is then placed on theRed Devil paint shaker and vigorously agitated for about minutes. Theblended mixture is then allowed to cool to approximately 78 F. and theviscosity of the blended solution is adjusted with tetrahydrofuran to 60centipoise seconds at 75 F. The solution/dispersion is then coated ontoa conductive substrate within 1 hour after the pigment paste has beenadded to the organic photoconductive charge transfer complex solution.

The above procedure will result in a solvent ratio of about 83 percenttetrahydrofuran and 17 percent toluene prior to adjusting the viscosity.After viscosity adjustment, the solvent ratio will generally be in therange of 84 to 88 percent tetrahydrofuran and approximately 16 to 12percent toluene. The toluene content of the final blend may be up toabout 30 percent of the solvent and still produce improved results. Theabove procedure was performed with a polyvinylcarbazole designatedLuvican M170 acquired from Badische Analin & Soda-Fabrik AG.

A photoconductive structure was made by coating a substrate with atleast one electrically conductive face with a layer of the aboveformulation. This structure was negatively charged by a corona and thequantity of light necessary to discharge the charged photoconductivestructure to a 200 volt charge level was measured. The exposure was theconverted to a relative sensitivity advantage expression resulting in asensitivity advantage of from 2.31: to 2.5x, this figure being arelative measure of sensitivity as compared to a photoconductor withoutchlorodiane blue pigment and using only a tetrahydrofuran solvent,meaning that the sensitivity was 2.3 to 2.5 times that of the standardwhich has a value of 1.0x.

EXAMPLE 2 A photoconductor coating liquid was prepared by adding to anabrasion resistant container 200 grams of clean steel shot, 57.81 gramsof tetrahydrofuran and 57.81 grams of toluene. To this solvent blend,9.38 grams of Formvar 15/95S, a polyvinylformaldehyde resin fromMonsanto Chemical Company, is added to the solvent blend and thecontainer is agitated on a Red Devil paint shaker for approximately 20minutes. After a 20 minute agitation on the Red Devil paint shaker, 7.37grams of chlorodiane blue pigment is added to the container and thecontainer is again agitated on the Red Devil paint shaker forapproximately 75 minutes. The container is removed from the shaker andimmediately 63.6 grams of the solvent/adhesive/pigment paste is added to250 grams of a 1:1 molar ratio PVCz/T NF solution having 28 percentsolids. This mixture is placed on the Red Devil paint shaker for 15minutes and the solution is then allowed to cool to approximately 78 F.This mixture is then adjusted in viscosity with tetrahydrofuran solventto 41 centipoise seconds at 75 F. This solution is then coated onto aconductive substrate within 1 hour after the paste has been added to theorganic photoconductive charge transfer complex solution.

The polyvinylcarbazole used in this example was secured from the DeSotoChemical Company. The differences between Example 1 and Example 2 areprimarily the stage at which the adhesive is added and the viscosity towhich the solution is adjusted. The above procedure will result in asolvent ratio of approximately 87 percent THF and 13 percent tolueneprior to viscosity adjustment and approximately 90 to 94 percent THF and10 to 6 percent toluene after the viscosity has been adjusted.

With respect to solids, the pigment concentration will be 4.5 weight percent and the adhesive concentration will be 6.0 weight percent.

The solution is coated onto an electrically conductive substrate,negatively charged by a corona, and the quantity of light necessary todischarge the charged photoconductive structure to a 200 volt chargelevel, measured.

This exposure, is converted into a relative sensitivity advantage, whichis based upon a photoconductive structure as discussed in FIG. 1. Therelative speed advantage is from about 1.98x to about 2.24x.

EXAMPLES 3 THROUGH 9 Photoconductive structures were formulated andconstructed according to the procedure of Example 2, using thepolyvinylcarbazole acquired from DeSoto Chemical Company and the Formvar15/95S adhesive. The only deviation from the procedure of Example 2 wasthe substitution of various alkyl substituted benzene solvents toillustrate that results approximately equal to or better than thatsecured by the use of pure tetrahydrofuran can be secured in the case ofall alkyl substituted benzene solvents tested which have a melting pointless than 20 C. and a boiling point of less than approximately 250 C.The results of the formulations are tabularly set forth in Table 1,together with repetitive results of Examples 1 and 2.

TABLE I Example Relative Coat wt. Residual No. Solvent Sensitivity mg/inSolvent 1 Toluene 2.38X N/A 0.18 1a Toluene 2.32X N/A 0.25 lb Toluene2.58X N/A 0.21 2 Toluene 2.24X 9.7 0.55 221 Toluene 1.98X 11.7 1.08 2bToluene 2.18X 13.4 0.68 2c Toluene 2.01X 10.6 0.68 2d Toluene 2.10X 10.60.46 2c Toluene 2.16X 10.5 0.41 3 Benzene 1.60X 9.7 0.15 4 Ethyl benzene1.80X 10.6 0.52 5 Xylene 1.54X 10.8 0.865 6 Propylbenzene 1.99 X 8.70.405 7 Mesitylene 1.97X 9.9 1.375 8 Tertiary butyl 1.50X 8.4 1.245

benzene 9 Cumene 1.67 12.9 1.02

Examples la and 1b, and 2a through 2e are repetitive tests using theprocedures of Examples 1 and 2 respectively.

"Used in Addition to THF As Per Examples 1 and 2.

"Of The Alkyl Substituted Benzene Solvent.

N/A Not Available.

Table 11 illustrates the results using only THF in the two procedures ofExamples 1 and 2, for comparative purposes.

TABLE 11 Relative Residual Procedure Sensitivity Solvent (THF) Example I1.54X 0.58 1.6OX 0.25 Example ll 1.52X 0.18

As can be seen from the tables above, seven additional solvents wereutilized and tested in addition to toluene and the results indicate thatthe sensitivity of a photoconductive structure formulated using thetechnique of Example 2 andthe various solvents, yields a photoconductorwhich is at least approximately as sensitive or more sensitive than thestraight tetrahydrofuran solvent system. It also appears that from thedifferent tests using the toluene, the preferred solvent is toluene butat least some of the other solvents such as propylbenzene, mesitylene,and ethylbenzene have a substantial sensitivity advantages over a'puretetrahydrofuran solvent. Differences of 0.1 or more between relativesensitivity figures are considered significant.

The residual solvent levels indicated in the table above, are theresidual solvent levels for the solvent indicated in the leftmost columnof Table I and tetrahydrofuran in Table II. The tests showed nodetectable tetrahydrofuran residual solvent in any of the samplestabulated in Table I. It appears that when a solvent blend oftetrahydrofuran and an alkyl substituted benzene is used and the alkylsubstituted benzene is used in a range of up to about percent andpreferably between about 6 to about 16 percent, any residual alkylsubstituted benzene solvent level below about 1.4 percent isinsufficient to exhibit a correlation between that and enhancedphotoconductive properties. It appears that by the use of the solventblend, the tetrahydrofuran may be virtually eliminated from theresulting photoconductor structure and a relatively low and stablequantity of residual solvent of the alkyl substituted benzene group mayremain in the photoconductor and have little if any effect on therelative sensitivity of the photoconductor.

Another pigment tested in the co-solvent system, diane blue, showedsensitivity advantage over the standard and also over the pigmentedphotoconductor formulated using tetrahydrofuran alone. The relativespeedadvantage over the standard, was 1.7X. No residual solvent data isavailable with respect to this formulation.

As can be seen from the foregoing, the use of an alkyl substitutedbenzene solvent in a proportion of approximately 50 percent to 100percent of the solvent used in milling the disazo pigment andparticularly chlorodiane blue pigment into a paste is advantageous. Thisimprovement yields an ultimate solvent blend of from 6 to about 16percent alkyl substituted benzene and from about 84 to 94 percenttetrahydrofuran. This photoconductor results in most cases in improvedlight sensitivity over the similar type structure which was constructedusing only a tetrahydrofuran solvent.

By using the solvent blend techniques as set forth above, it is believedthat the light sensitivity of the photoconductive structure will beenhanced to a point where it more nearly approaches the potential lightsensitivity of the chlorodiane blue pigmented coating.

The differences between Example 1 and Example 2 above, particularly inthe formulating technique and the viscosity adjustment is due, it isbelieved, to the difference in the molecular weights between the LuvicanM170 and the DeSoto Chemical Company polyvinylcarbazole. Secondly, ithas been found that the Formvar will dissolve in the THF but ispartially incompatible with the PVCzzTNF complex. Therefore, it isformed as part of the pigment paste necessitating the use of a solventblend to adequately dissolve the material and accomplish the objects ofthis invention.

The following invention has been particularly shown and described withreference to preferred embodiments thereof, to be understood by thoseskilled in the art that the foregoing and other changes in form anddetail may be made therein without departing from the spirit and scopeof the invention.

Iclaim:

1. An electric photoconductive member comprising:

a substrate with at least one electrically conductive surface;

a photoconductive matrix medium overlying and in electrical contact withsaid electrically conductive surface;

' a disazo pigment dispersed in particulate form throughout saidphotoconductive matrix medium; the improvement comprising:

said photoconductive matrix medium together with said disazo pigmentparticles,-bei'ng the residue of a coating solution/dispersioncomprising a charge transfer complex of polyvinyl-carbazole andtrinitrofluorenone dissolved in tetrahydrofuran; and

a sensitizing quantity of said disazo pigment, said pigment having beenmilled to submicron size particles in a solvent comprising an alkylsubstituted benzene, said alkyl substituted benzene being liquid atbelow about 20 C. and having a boiling point of less than about 250 C.

2. The photoconductive structure of claim 1, wherein said disazo pigmentis chlorodiane blue.

3. The photoconductive structure of claim 1, wherein said alkylsubstituted benzenes are selected from the group consisting of benzene,toluene, ethylbenzene, xylene, propylbenzene, mesitylene, tertiarybutylbenzene, and cumene.

4. The photoconductive structure of claim 2 wherein said chlorodianeblue is present in a percentage of from about 1 to about 8 weightpercent.

5. The photoconductive structure of claim 1, wherein said alkylsubstituted benzene solvent is present in the solution/dispersion, priorto coating, in an amount up to about 30 weight percent of said solvents.

said alkyl substituted benzene is ethylbenzene.

2. The photoconductive structure of claim 1, wherein said disazo pigmentis chlorodiane blue.
 3. The photoconductive structure of claim 1,wherein said alkyl substituted benzenes are selected from the groupconsisting of benzene, toluene, ethylbenzene, xylene, propylbenzene,mesitylene, tertiary butylbenzene, and cumene.
 4. The photoconductivestructure of claim 2, wherein said chlorodiane blue is present in apercentage of from about 1 to about 8 weight percent.
 5. Thephotoconductive structure of claim 1, wherein said alkyl substitutedbenzene solvent is present in the solution/dispersion, prior to coating,in an amount up to about 30 weight percent of said solvents.
 6. Thephotoconductive structure of claim 1, wherein said alkyl substitutedbenzene is toluene.
 7. The photoconductive structure of claim 1, whereinsaid alkyl substituted benzene is propylbenzene.
 8. The photoconductivestructure of claim 1, wherein said alkyl substituted benzene ismesitylene.
 9. The photoconductive structure of claim 1, wherein saidalkyl substituted benzene is ethylbenzene.